Albert Barnes on Matthew 5:33

Thou shalt not forswear thyself. Christ here proceeds to correct another false interpretation of the law. The law respecting oaths is found in Lev 19:12, De 23:23. By those laws, men were forbid to perjure themselves, or to forswear, that is, swear falsely.

Perform unto the Lord. Perform literally, really, and religiously, what is promised in an oath.

Thine oaths. An oath is a solemn affirmation, or declaration, made with an appeal to God for the truth of what is affirmed, and imprecating his vengeance, and renouncing his favour, if what is affirmed is false. A false oath is called perjury; or, as in this place, forswearing.

It appears, however, from this passage, as well as from the ancient writings of the Jewish Rabbins, that while they professedly adhered to the law, they had introduced a number of oaths in common conversation, and oaths which they by no means considered as binding. For example, they would swear by the temple, by the head, by heaven, by the earth. So long as they kept from swearing by the name Jehovah, and so long as they observed the oaths publicly taken, they seemed to consider all others as allowable, and allowedly broken. This is the abuse which Christ wished to correct. It was the practice of swearing in common conversation, and especially swearing by created things. To do this, he said that they were mistaken in their views of the sacredness of such oaths. They were very closely connected with God; and to trifle with them was a species of trifling with God. Heaven is his throne; the earth his footstool; Jerusalem his peculiar abode; the head was made by him, and was so much under his control, that we could not make one hair white or black. To swear by these things, therefore, was to treat irreverently objects created by God; and could not be without guilt

Our Saviour here evidently had no reference to judicial oaths, or oaths taken in a court of justice. It was merely the foolish and wicked habit of swearing in private conversation; of swearing on every occasion, and by everything, that he condemned. This he does condemn in a most unqualified manner. He himself, however, did not refuse to take an oath in a court of law, Mt 26:63,64. So Paul often called God to witness his sincerity, which is all that is meant by an oath. See Rom 1:9, 9:1, Gal 1:20, Heb 6:16. Oaths were, moreover, prescribed in the law of Moses, and Christ did not come to repeal those laws. See Ex 22:11, Lev 5:1, Nu 5:19 De 29:12,14.

(z) "not forswear thyself" Lev 19:12, Nu 30:2, De 23:23